Birefringent films comprising polymer liquid crystal material are known in prior art. GB 2 324 382 discloses a liquid crystal polymer film with homeotropic alignment prepared from a polymerisable liquid crystal material that is coated onto a plastic substrate and polymerised, whereby roll to roll coating is applicable. The use of an aligning layer comprising a surfactant being fixed in a matrix of a polymeric liquid crystalline material is also described. According to an alternative embodiment the substrate is coated with aluminium or sufficiently smooth Al2O3 to achieve a homeotropic alignment of the liquid crystal material. The resulting uniformly homeotropic aligned films can be used in electrooptical displays, optical compensation layers or as active switching layer.
The use of birefringent films as security devices has also been reported in prior art.
GB 2 330 360 describes a polymer film with a helically twisted molecular structure exhibiting at least two maxima of a reflection wavelength. The film is obtained by polymerizing a thermochromic mesogenic composition at varied temperatures. Since the temperature is changed during polymerization, the thermochromic polymerizable composition undergoes a change of the helical pitch and therewith of the reflection maximum. The various pitch lengths are fixed by the polymerization reaction and a polymer film is obtained that exhibits, depending on the type, degree and speed of temperature variation, a range of reflection colours.
However, a disadvantage of the method described in GB 2 330 360 is the requirement of a precise control of the temperature and its change during the polymerization process, and the need of specific materials with thermochromic properties.
GB 2 357 061 discloses a hot stamping foil for security applications comprising a layer of polymerised liquid crystal material. The liquid crystal polymer layer is prepared on a plastic substrate and then laminated with a reflective layer of e.g. aluminium which exhibits on its opposite surface a hot melt adhesive. The liquid crystal layer is protected by a lacquer. The whole arrangement of layers is carried by a support layer. A wax layer between the support layer and the lacquer layer enables a release of the birefringent layer arrangement by applying heat. The liquid crystal material is for example a polymerised or crosslinked nematic or smectic material with a planar, tilted, splayed or homeotropic structure and macroscopically uniform orientation. The hot stamping foil can be used for decorative purposes or to authenticate and prevent counterfeiting of documents of value, e.g. of banknotes, credit cards or ID cards. The transmission or reflection effect of the polarized light by the birefringent material can be seen using linear polarizers. Incorporation of dichroic dyes leads to additional color effects.
However, the process described in GB 2 357 061 requires the additional process step of transferring or laminating the polymeric liquid crystalline material, which is typically prepared on a plastic substrate, to the reflective aluminium layer, or alternatively applying the aluminium layer onto the liquid crystal layer. This is required since the natural alignment of nematic liquid crystals on aluminium surfaces is homeotropic. However, this alignment would not be useful for displaying hidden designs. Also, the preparation of a liquid crystal polymer layer by coating a polymerisable material onto a substrate usually yields uniform coated areas of the material. The formation of images or patterns would require specific materials or additional techniquies such as photomasking or photoorientation.